1. Field of the Invention
The present invention pertains to wireless telecommunications, and particularly to recovery when there has been a loss of certain information at a node of a radio access network that controls base stations.
2. Related Art and Other Considerations
In a typical cellular radio system, mobile user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM-based radio access network technologies.
As those skilled in the art appreciate, in W-CDMA technology a common frequency band allows simultaneous communication between a user equipment unit (UE) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo-noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the user equipment units (UEs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. The high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal. In CDMA, therefore, a user equipment unit (UE) need not switch frequency when handoff of a connection is made from one cell to another. As a result, a destination cell can support a connection to a user equipment unit (UE) at the same time the origination cell continues to service the connection. Since the user equipment unit (UE) is always communicating through at least one cell during handover, there is no disruption to the call. Hence, the term “soft handover.” In contrast to hard handover, soft handover is a “make-before-break” switching operation.
There are several interfaces of interest in the UTRAN. The interface between the radio network controllers (RNCs) and the core network(s) is termed the “Iu” interface. The interface between a radio network controller (RNC) and its base stations (BSs) is termed the “lub” interface. The interface between the user equipment unit (UE) and the base stations is known as the “air interface” or the “radio interface” or “Uu interface”. In some instances, a connection involves both a Serving or Source RNC (SRNC) and a target or drift RNC (DRNC), with the SRNC controlling the connection but with one or more diversity legs of the connection being handling by the DRNC. An Inter-RNC transport link can be utilized for the transport of control and data signals between Source RNC and a Drift or Target RNC, and can be either a direct link or a logical link. An interface between radio network controllers (e.g., between a Serving RNC [SRNC] and a Drift RNC [DRNC]) is termed the “lur” interface.
The radio network controller (RNC) controls the UTRAN. In fulfilling its control role, the RNC manages resources of the UTRAN. Such resources managed by the RNC include (among others) the downlink (DL) power transmitted by the base stations; the uplink (UL) interference perceived by the base stations; and the hardware situated at the base stations.
Those skilled in the art appreciate that, with respect to a certain RAN-UE connection, an RNC can either have the role of a serving RNC (SRNC) or the role of a drift RNC (DRNC). If an RNC is a serving RNC (SRNC), the RNC is in charge of the connection with the user equipment unit (UE), e.g., it has full control of the connection within the radio access network (RAN). A serving RNC (SRNC) is connected to the core network. On the other hand, if an RNC is a drift RNC (DRNC), its supports the serving RNC (SRNC) by supplying radio resources (within the cells controlled by the drift RNC (DRNC)) needed for a connection with the user equipment unit (UE). A system which includes the drift radio network controller (DRNC) and the base stations controlled over the Jub Interface by the drift radio network controller (DRNC) is herein referenced as a DRNC subsystem or DRNS. An RNC is said to be the Controlling RNC (CRNC) for the base stations connected to it by an Iub interface. This CRNC role is not UE specific. The CRNC is, among other things, responsible for handling radio resource management for the cells in the base stations connected to it by the Iub interface.
The UTRAN interfaces (Iu, Iur and Iub) have two planes, namely, a control plane (CP) and a user plane (UP). In order to control the UTRAN, the radio network application in the different nodes communicate by using the control plane protocols. The RANAP is a control plane protocol for the Iu interface; the RNSAP is a control plane protocol for the lur interface; and NBAP is a control plane protocol for the Iub interface. The control plane protocols are transported over reliable signaling bearers. The transport of data received/transmitted on the radio interface occurs in the user plane (UP). In the user plane, the data is transported over unreliable transport bearers. The serving radio network controller (SRNC) is responsible for establishing the necessary transport bearers between the serving radio network controller (SRNC) and the drift radio network controller (DRNC).
Operation of a user equipment unit (UE) is conceptualized from the perspective of radio resource control (RRC) as having two modes: an Idle Mode and a Connection Mode. FIG. 10 shows a state model relevant to a user equipment unit (UE) having these two modes. The Idle Mode is entered after power on. In Idle Mode there is no connection between the user equipment unit (UE) and the UTRAN. When a connection is established, the user equipment unit (UE) is assigned a U-RNTI and the user equipment unit (UE) enters Connected Mode. The U-RNTI (UTRAN Radio Network Temporary Identity) is a global identity, which can be used in any cell in the UTRAN.
Within Connected Mode there are four different states: CELL_DCH state; CELL_FACH state; CELL_PCH state; and URA_PCH . As briefly described below, each state reflects a different level of activity.
The CELL_DCH state is characterized by that there is a dedicated channel (DCH) assigned to the user equipment unit (UE). Macro-diversity may be used between DCHs of several cells. In the CELL_DCH state, there is a dedicated control channel (DCCH) used for transmission of signalling messages between the user equipment unit (UE) and the UTRAN.
In the CELL_FACH state, no dedicated physical channel is assigned, but the user equipment unit (UE) listens continuously to a common channel (the FACH) in the downlink belonging to the selected cell. In the uplink, the user equipment unit (UE) typically uses a random access channel (RACH). At each cell reselection, the user equipment unit (UE) updates the network with its current cell location. In this state, there is a dedicated control channel (DCCH) used for transmission of signalling messages between the user equipment unit (UE) and the UTRAN. The DCCH is implemented by appending the Radio Network Temporary Identity (U-RNTI or C-RNTI) to all signalling messages, and thus addressing an individual UE. As mentioned previously, the U-RNTI (UTRAN RNTI) is a global identity, which can be used in any cell in the UTRAN. The C-RNTI (Cell RNTI) is only significant in a single cell, and has to be reallocated in every cell. On the other hand, C-RNTI is much shorter than the U-RNTI which saves space over the radio interface when it is used. There is also a CCCH (Common control channel) in this state, which is used when the connection to the SRNC is not available, such at after cell reselection over RNC borders, when the CELL UPDATE or URA UPDATE message is sent to the DRNC.
In the CELL_PCH state, the user equipment unit (UE) monitors a paging channel (PCH) of a selected cell. On the PCH, the user equipment unit (UE) uses discontinuous reception (DRX) to save power, and the scheme for when to listen is agreed between the network and the user-equipment unit (UE) on a per user equipment unit (UE) basis. Also in the CELL_PCH state the user equipment unit (UE) updates the network with its current cell location at cell reselection. No DCCH is available in the CELL_PCH state. On the PCH, means for addressing individual user equipment units (UEs) exist (using the U-RNTI), but the user equipment unit (UE) can not transport any signalling messages to the network.
The URA_PCH state is almost identical to the CELL_PCH state. The difference is that the user equipment unit (UE) does only update the network of its location after crossing URA borders. An URA (UTRAN Registration Area) is a group of cells. This means that in this state the position of the user equipment unit (UE) is in general known only on URA level.
The user equipment units (UEs) in the states CELL_PCH and URA_PCH of the RRC Connected Mode listen to the RRC Page type 1 (see, 3GPP TS 25.331, RRC Protocol Specification). In the RRC Page type 1, the paged UE is addressed by the UTRAN Identity (U-RNTI). As shown by FIG. 11, the U-RNTI actually comprises two portions, particularly a twelve bit SRNC-id information element portion and a twenty bit S-RNTI information element portion. The SRNC-id information element is typically an identifier of the SRNC which serves the user equipment unit (UE). The S-RNTI information element is a number allocated by that SRNC for distinguishing the user equipment unit (UE) within that SRNC.
When a user equipment unit (UE) is in the Idle mode, on the other hand, the CN Identity (TMSI) is used for paging. The TMSI (temporary mobile station identifier) is typically assigned to a user equipment unit (UE) while the user equipment unit (UE) is in a certain multicell area. Unfortunately, the CN Identity (TMSI) can not be used for paging when the user equipment unit (UE) is in the states CELL_PCH and URA_PCH of the RRC Connected Mode. This is because the CN Identity (TMSI) is only valid in the Location Area where the user equipment unit (UE) first established the RRC Connection and performed Location Update. After connection establishment and Location Update, if the user equipment unit (UE) being in the Connected Mode moves to another Location Area (LA), no further Location Update procedure occurs (since a Location Update procedure is not performed while the user equipment unit (UE) is in its Connected Mode). Therefore, in the Connected Mode the UTRAN Identity (U-RNTI) is used for paging the user equipment unit (UE) instead of the CN Identity (TMSI).
Unfortunately, after a failure of a radio network controller (RNC) node, some relevant information about the user equipment units that are connected to the RNC is lost. This will result in those user equipment units not being reached by the UTRAN. For instance, if the lost information includes the U-RNTI of a user equipment unit in the connected mode, paging of that user equipment unit is not possible. This will result in the user equipment unit being a “hanging UE” until the user equipment unit next performs a location update (e.g., either a periodic cell update or a periodic URA update). Such periodic updates (either the periodic cell update or the periodic URA update) are prompted by expiration of appropriate timers, e.g., a periodic cell update timer or a periodic URA update timer. However, the expiration times of these timers are typically quite long, on the order of thirty minutes to an hour. Until performance of either the periodic cell update or the periodic URA update, as appropriate, the affected 30 user equipment unit cannot be reached. While one remedial measure might be to decrease the expiration time of the timers, such decrease would result in a corresponding undesirable increased signaling load (due to increased periodic updates) during the normal operation.
After such RNC failure, there can be paging initiated by the Core Network (CN) and sent by UTRAN over the Uu interface (e.g., radio interface), but such paging will be addressed using the CN Identity (TMSI). However, user equipment units (UEs) in the CELL_PCH and URA_PCH states will simply disregard such attempted paging.
Thus, in the case of the partial loss of the user equipment unit (UE) context (that is, the UE context information (U-RNTI) is lost for some user equipment units) the user equipment units for which the UE context has been lost will themselves essentially be lost and can not be paged. For those user equipment units, in the current radio access network schemes there is no possibility of regaining the user equipment units, but rather only the drastic measures of turning off the transmission in the cells or barring the cells. Such drastic measures can, in turn, negatively affect other user equipment units whose context information was not lost.
The aforementioned problem arises in several situations. For example, the problems arises when the SRNC and CRNC are one and the same. The problem can also arise when the serving RNC (SRNC) is not the controlling RNC of the cells where the user equipment unit (UE) is currently located, i.e. the user equipment unit is connected to SRNC via Iur interface. In the case of the SRNC reset, there is no mechanism to inform the user equipment units in these states that the SRNC is not reachable anymore or their contexts need to be updated.
What is needed, therefore, and an object of the present invention, is a technique for recovering the otherwise lost (e.g., hanging) user equipment units in certain critical states of the Connected Mode after loss of the context information (e.g., U-RNTI) of user equipment unit, as can occur after an RNC failure.